This invention relates to compositions useful for forming conversion coatings on metal surfaces. In particular, this invention pertains to aqueous compositions which, when dried in place on a metallic surface comprised of aluminum, provide a coating having reduced odor characteristics.
It is well known to treat aluminum and aluminum alloy surfaces with various agents for purposes of rendering the surfaces more resistant to corrosion and enhancing the adherence of top coats and bonded films to such surfaces. Additionally, there are situations in which it will be desirable to impart hydrophilic characteristics to the aluminum-containing surface. For example, aluminum and its alloys are often used to fabricate heat exchangers having a large surface area so as to maximize the heat exchangers"" heat dissipating and cooling capacity. In order to minimize the size of the heat exchanger, however, it will generally be desirable to reduce the space between the individual fins of the heat exchanger. If water drops are capable of forming on the heat exchanger surface, this tends to reduce the effectiveness of the heat exchanger since air is no longer able to freely circulate within or pass through the heat exchanger. Increasing the hydrophilicity (xe2x80x9cwettabilityxe2x80x9d) of the heat exchanger surface will allow such droplets to spread out.
As heat exchangers are often used in air conditioning units wherein air is drawn into such a unit and then expelled into occupied spaces, it is important that the coating does not impart any objectionable odor to the conditioned air. Unfortunately, many of the known technologies for forming corrosion resistant, hydrophilic conversion coatings on aluminum surfaces yield coatings which have a disagreeable or unpleasant smell, particularly when wet. Additionally, certain of these technologies utilize chromium compounds which are undesirable in view of their known environmental and toxicological properties. Further, certain of these technologies employ polymers or other organic substances having rather complex structures and which are therefore often relatively expensive to purchase or synthesize.
Thus, it is apparent that there is still a great need for a process capable of forming a hydrophilic, corrosion-resistant coating on metal surfaces (especially aluminum surfaces) which does not require the use of either chromium compounds or polymeric or other organic substances and which furnishes a coating having little or no odor.
The present invention provides a composition useful for forming conversion coatings. The composition comprises water, at least one fluorometallate of at least one element selected from the group consisting of Ti, Zr, Si, B, Hf, Sn and Ge, and at least one vanadium compound. In an especially desirable embodiment, the composition is additionally comprised of at least one oxide, hydroxide, carbonate or alkoxide of at least one element selected from the group consisting of Ti, Zr, Si, B, Hf, Sn, Al and Ge or a salt thereof A conversion coating is formed by contacting such composition with the surface of a metal, particularly a metal comprised of aluminum. The composition may thereafter be dried in place without rinsing.
The conversion coatings thus formed are relatively low in odor (especially when the aforedescribed oxide, hydroxide, carbonate or alkoxide component is additionally present in the composition used to form the coating). Moreover, advantageous corrosion resistance and hydrophilic characteristics are imparted to the metal surface without the use of either chromium compounds or polymeric or other organic substances.